Metrology system with spectroscopic ellipsometer and photoacoustic measurements

ABSTRACT

An optical system includes both a microspot broadband spectroscopic ellipsometer and a photoacoustic film thickness measurement system that are supplied laser light by the same laser light source. One of the systems makes a measurement, the result of which is used to adjust a parameter of the other system; e.g. the ellipsometer measures thickness and the photoacoustic system uses the thickness result to measure the speed of sound. In one version, the ellipsometer converts the laser beam to a broad-spectrum beam that provides higher intensity.

CLAIM OF PRIORITY FROM PROVISIONAL PATENT APPLICATION

This patent application claims priority under 35 U.S.C. §119(e) fromProvisional Patent Application No.: 60/305,277, filed Jul. 13, 2001, andfrom Provisional Patent Application No.: 60/306,120, filed Jul. 17,2001, the disclosures of which are incorporated by reference herein intheir entireties.

BACKGROUND OF THE INVENTION

The semiconductor processing industry has made significant progress inrecent years in forming ever-smaller minimum device geometries, whichhas created a need for processes that form very thin films. Thisdevelopment has in turn led to a need for metrology equipment to measurethose films. In many process steps, the thickness of the thin films usedto form these structures is becoming ever thinner. Gate oxidethicknesses, for example are now typically on the order of 10 to 20Angstroms thick. One technique for measuring the thickness of such filmsis known as ellipsometry.

Ellipsometry is a non-contact, nondestructive, optical technique for thecharacterization of transparent thin films on surfaces. When a surfaceor interface is struck by polarized light, ellipsometers measure thechange in the reflected light's polarization by detecting andquantifying the change in the amplitude ratio (psi) and the change inphase (delta) induced by reflection of light from the surface.

In another trend, the increasing requirements of high-speed andlow-power semiconductor devices has resulted in a significant shift awayfrom aluminum conductors and silicon oxide insulators as the dominantmetal/insulator combination in semiconductor multi-level metallizationtechnology. Copper and low k dielectric materials are replacing aluminummetallurgy and silicon oxide dielectrics. It is also anticipated thatcopper metallurgy and low k dielectric materials will dominate thesemiconductor integrated circuit designs. One technique for measuringthe thickness of metal films is known as photoacoustic film thicknessmeasurement.

Photoacoustic film thickness measurement is a non-contact,nondestructive optical technique for measuring the thickness of singleor multi-layer opaque metal films. A photoacoustic thickness measurementsystem forms two optical beams: an excitation beam used to excite thesurface of the film sample periodically, and a probe beam used to sensethe reflectivity of the surface of the sample following each pulse fromthe excitation beam.

Prior Art

Prior Art for Photoacoustic Film Thickness Measurement

U.S. Pat. No. 6,069,703 entitled “Method and device for simultaneouslymeasuring the thickness of multiple thin metal films in a multilayerstructure”, assigned to Active Impulse Systems, Inc., (Natick, Mass.),discloses an apparatus for measuring a property of a structurecomprising at least one layer, the apparatus including a light sourcethat produces an optical pulse having a duration of less than 10 ps; adiffractive element that receives the optical pulse and diffracts it togenerate at least two excitation pulses; an optical system thatspatially and temporally overlaps at least two excitation pulses on orin the structure to form an excitation pattern, containing at least twolight regions, that launches an acoustic wave having an out-of-planecomponent that propagates through the layer, reflects off a lowerboundary of the layer, and returns to a surface of the structure tomodulate a property of the structure; a light source that produces aprobe pulse that diffracts off the modulated property to generate atleast one signal pulse; a detector that receives at least one signalpulse and in response generates a light-induced electrical signal; andan analyzer that analyzes the light-induced electrical signal to measurethe property of the structure.

U.S. Pat. No. 6,008,906 entitled “Optical method for thecharacterization of the electrical properties of semiconductors andinsulating films”, assigned to Brown University Research Foundation,(Providence, R.I.), discloses a method for characterizing a sampleincluding the steps of (a) providing a semiconductor material; (b)applying at least one of an electric field, a pulsed or cw light source,a change in temperature and/or a change in pump pulse intensity to thesemiconductor material; (c) absorbing pump light pulses in a portion ofthe semiconductor material and measuring changes in optical constants asindicated by probe light pulses applied at some time t following theabsorption of the pump light pulses; and (e) associating a measuredchange in the optical constants with at least one of a surface charge,dopant concentration, trap density, or minority carrier lifetime.

U.S. Pat. No. 5,959,735 entitled “Optical stress generator anddetector,” assigned to Brown University Research Foundation,(Providence, R.I.), discloses a system for the characterization of thinfilms and interfaces between thin films through measurements of theirmechanical and thermal properties. In the system, light is absorbed in athin film or in a structure made up of several thin films, and thechange in optical transmission or reflection is measured and analyzed.The change in reflection or transmission is used to give informationabout the ultrasonic waves that are produced in the structure. Theinformation that is obtained from the use of the measurement methods andapparatus of this invention can include: (a) a determination of thethickness of thin films with a speed and accuracy that is improvedcompared to earlier methods; (b) a determination of the thermal,elastic, and optical properties of thin films; (c) a determination ofthe stress in thin films; and (d) a characterization of the propertiesof interfaces, including the presence of roughness and defects.

U.S. Pat. No. 5,748,318 entitled “Optical stress generator anddetector,” assigned to Brown University Research Foundation,(Providence, R.I.), discloses a system for the characterization of thinfilms and interfaces between thin films through measurements of theirmechanical and thermal properties. In the system, light is absorbed in athin film or in a structure made up of several thin films, and thechange in optical transmission or reflection is measured and analyzed.The change in reflection or transmission is used to give informationabout the ultrasonic waves that are produced in the structure. Theinformation that is obtained from the use of the measurement methods andapparatus of this invention can include: (a) a determination of thethickness of thin films with a speed and accuracy that is improvedcompared to earlier methods; (b) a determination of the thermal,elastic, and optical properties of thin films; (c) a determination ofthe stress in thin films; and (d) a characterization of the propertiesof interfaces, including the presence of roughness and defects.

Prior Art for Spectroscopic Ellipsometer

U.S. Pat. No. 5,978,074 entitled “Apparatus for evaluating metallizedlayers on semiconductors,” assigned to Therma-Wave, Inc., (Fremont,Calif.), discloses an apparatus for characterizing multilayer samples.An intensity modulated pump beam is focused onto the sample surface toperiodically excite the sample. A probe beam is focused onto the samplesurface within the periodically excited area. The power of the reflectedprobe beam is measured by a photodetector. The output of thephotodetector is filtered and processed to derive the modulated opticalreflectivity of the sample. Measurements are taken at a plurality ofpump beam modulation frequencies. In addition, measurements are taken asthe lateral separation between the pump and probe beam spots on thesample surface is varied. The measurements at multiple modulationfrequencies and at different lateral beam spot spacings are used to helpcharacterize complex multilayer samples. In the preferred embodiment, aspectrometer is also included to provide additional data forcharacterizing the sample.

U.S. Pat. No. 5,973,787 entitled “Broadband spectroscopic rotatingcompensator ellipsometer,” assigned to Therma-Wave, Inc., (Fremont,Calif.), discloses an ellipsometer, and a method of ellipsometry, foranalyzing a sample using a broad range of wavelengths, including a lightsource for generating a beam of polychromatic light having a range ofwavelengths of light for interacting with the sample. A polarizerpolarizes the light beam before the light beam interacts with thesample. A rotating compensator induces phase retardations of apolarization state of the light beam wherein the range of wavelengthsand the compensator are selected such that at least a first phaseretardation value is induced that is within a primary range of effectiveretardations of substantially 135° to 225°, and at least a second phaseretardation value is induced that is outside of the primary range. Ananalyzer interacts with the light beam after the light beam interactswith the sample. A detector measures the intensity of light afterinteracting with the analyzer as a function of compensator angle and ofwavelength, preferably at all wavelengths simultaneously. A processordetermines the polarization state of the beam as it impinges theanalyzer from the light intensities measured by the detector.

U.S. Pat. No. 5,910,842 entitled “Focused beam spectroscopicellipsometry method and system,” assigned to KLA-Tencor Corporation,(San Jose, Calif.), discloses a method and system for spectroscopicellipsometry employing reflective optics to measure a small region of asample by reflecting radiation (preferably broadband UV, visible, andnear infrared radiation) from the region. The system preferably has anautofocus assembly and a processor programmed to determine from themeasurements the thickness and/or complex refractive index of a thinfilm on the sample. Preferably, only reflective optics are employedalong the optical path between the polarizer and analyzer, a sample beamreflects with low incidence angle from each component of the reflectiveoptics, the beam is reflectively focused to a small, compact spot on thesample at a range of high incidence angles, and an incidence angleselection element is provided for selecting for measurement onlyradiation reflected from the sample at a single, selected angle (ornarrow range of angles). The focusing mirror preferably has anelliptical shape to reduce off-axis aberrations in the focused beam.Some embodiments include both a spectrophotometer and an ellipsometerintegrated together as a single instrument. In such instrument, thespectrophotometer and ellipsometer share a radiation source, andradiation from the source can be focused by either the spectrophotometeror the ellipsometer to the same focal point on a sample. Preferredembodiments of the ellipsometer employ a rotating, minimal-length Rochonprism as a polarizer, and include a spectrometer with an intensifiedphotodiode array to measure reflected radiation from the sample, and areference channel (in addition to a sample channel which detectsradiation reflected from the sample).

U.S. Pat. No. 5,900,939 entitled “Thin film optical measurement systemand method with calibrating ellipsometer,” assigned to Therma-Wave,Inc., (Fremont, Calif.), discloses an optical measurement system forevaluating a reference sample that has at least a partially knowncomposition. The optical measurement system includes a referenceellipsometer and at least one non-contact optical measurement device.The reference ellipsometer includes a light generator, an analyzer and adetector. The light generator generates a beam of quasimonochromaticlight having a known wavelength and a known polarization for interactingwith the reference sample. The beam is directed at a non-normal angle ofincidence relative to the reference sample to interact with thereference sample. The analyzer creates interference between the S and Ppolarized components in the light beam after the light beam hasinteracted with reference sample. The detector measures the intensity ofthe light beam after it has passed through the analyzer. A processordetermines the polarization state of the light beam entering theanalyzer from the intensity measured by the detector, and determines anoptical property of the reference sample based upon the determinedpolarization state, the known wavelength of light from the lightgenerator and the composition of the reference sample. The processoralso operates the optical measurement device to measure an opticalparameter of the reference sample. The processor calibrates the opticalmeasurement device by comparing the measured optical parameter from theoptical measurement device to the determined optical property from thereference ellipsometer.

U.S. Pat. No. 6,052,188 entitled “Spectroscopic ellipsometer,” assignedto Verity Instruments, Inc., (Carrollton, Tex.), discloses a spectralellipsometer that enables complete simultaneous measurement ofellipsometric parameters of a surface with thin films and coatings forthe full wavelength range of interest by using an imaging spectrographtogether with a novel optical arrangement that disperses thepolarization information of a time-invariant train of optical signals ina linear spatial array of points along or parallel to an input apertureor slit of the imaging spectrograph and disperses the polarizationinformation in wavelength perpendicular to the aperture or slit toprovide a two-dimensional spectrograph image that is collected andstored by an imaging array with one axis relating to wavelength and theother axis relating to the light polarization. Multiple simultaneousmeasurements of the spectral ellipsometric parameters psi and delta aretaken at all wavelengths without the need of any time-varying ormechanically-moving optical elements. The ellipsometer can be used forreal-time measurements of ellipsometric parameters of a moving or staticsurface with the thin films and coatings.

U.S. Pat. No. 5,329,357 entitled “Spectroscopic ellipsometry apparatusincluding an optical fiber,” assigned to Sopra-Societe De Production EtDe Recherches Appliquees, (Bois-Colombes, FR), discloses a spectroscopicellipsometer comprises a wideband light source, together with a firstoptical system including a rotating polarizer which applies a parallelbeam to a sample contained in an enclosure. The reflected beam is pickedup by an analyzer in a second optical system which transmits saidreflected beam to a monochromator which is followed by a photodetectorwhich is connected to control electronics connected, in turn, to amicrocomputer. An optical fiber is provided between the source and thefirst optical system. Advantageously, a second optical fiber providedbetween the second optical system and the monochromator.

Since higher speed and higher precision thickness measurement boosts thethroughput and yield of semiconductor processing lines, which in turncontribute significant economic benefit to semiconductor manufacturers,there is a strong demand for such improvements in the industry.Likewise, there is also a strong demand in the industry for systems withthe flexibility to handle a broad range of measurement requirements.

Semiconductor device fabrication plants house numerous pieces ofequipment, each having their own space requirements for installation.Since the space required by each piece of equipment in a plantcontributes directly to the total overhead cost of the plant, it isdesirable to reduce the total space requirement of a plant by combiningand integrating the functions of multiple pieces of equipment into onepiece of equipment.

In the field of metrology, ellipsometers measure the thickness oftransparent films, and photoacoustic film thickness measurement systemsmeasure the thickness of opaque films. However, these separate pieces ofequipment each require their own installation space in a semiconductorfabrication plant, which leads to a high overhead cost of the plant.What is needed is a way to provide a dual system for measuring thethickness of transparent and opaque films.

SUMMARY OF THE INVENTION

The invention relates to a dual metrology system having both aspectroscopic ellipsometer and a photoacoustic film thicknessmeasurement system.

A feature of the invention is that the spectroscopic ellipsometer andphotoacoustic measurement system are supplied laser light by the samelaser light source.

Another feature of the invention is that the spectroscopic ellipsometeris a microspot broadband spectroscopic ellipsometer.

Yet another feature of the present invention is a method of measuringthe thickness of a film on a wafer comprising the steps of positioningin the measurement area a selected site on a film formed on a sample;using either the ellipsometer or photoacoustic system to make ameasurement on the selected site; and using the result of thatmeasurement to adjust a parameter of the other system.

Yet another feature of the present invention is a method in which theellipsometer is used to calculate the optical constants of an opaquefilm to improve the model for calculating the film thickness using thephotoacoustic system.

Yet another feature of the present invention is a method in which boththe ellipsometer and photoacoustic system are used in the steps tomeasure, calculate and report the film thickness, the results of themeasurements being combined to provide an improved result.

Yet another feature of the invention is a method in which bothmeasurements of the ellipsometer and photoacoustic system are used tocalculate the combined transparent and opaque thickness of a materialfilm stack consisting of one or more transparent films lying over one ormore opaque films.

Yet another feature of the invention is a method in which theellipsometer is used to calculate the transparent film thickness, andthe photoacoustic system is used to calculate the speed of sound in thefilm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically a prior art measurement system.

FIG. 2 illustrates a prior art ellipsometry system.

FIG. 3 illustrates schematically a prior art photo-acoustic measurementsystem.

FIG. 4 illustrates schematically a measurement system according to theinvention.

FIG. 5 illustrates a plan view of a measurement system according to theinvention.

FIG. 6 illustrates a plan view of an alternative embodiment of ameasurement system according to the invention.

FIG. 7 illustrates steps in a method of a measurement according to theinvention.

FIG. 8 illustrates a broadband component of an ellipsometer.

FIG. 9 illustrates a broadband spectrum for an ellipsometer.

DESCRIPTION OF THE INVENTION

FIG. 1 shows a block diagram of a generalized prior art metrologysystem. Metrology system 100 comprises a measurement system 110, acontroller 120, communication lines 130, a cassette station 140, arobotics & wafer handling system 150, and a measurement stage 160.

Measurement system 110 is either an ellipsometer or a photoacousticsystem, as described in detail below. Measurement stage 160 comprisestranslation stages to position a wafer in a desired location beneathmeasurement system 110, and a translation stage to move the wafer towardor away from measurement system 110. Robotics and wafer handling system150 comprises wafer gripping mechanisms, robots, and robotic controllersystem hardware and software to facilitate the transport of wafers fromone location to another. Communication lines 130 are standardcomputer-to-instrument interface wires, fiber-optic cables, wireless,etc. Controller 120 comprises a computing device with a processor andmemory. Controller 120 is electrically connected via communication lines130 to measurement system 110, cassette station 140, robotics and waferhandling system 150, and measurement stage 160.

FIG. 2 shows the configuration of a typical prior art simultaneousmultiple-angle spectroscopic ellipsometer. Ellipsometer 200 includes alight source 205 for generating a light beam 210, which passes throughbeam shaping optical system 215, polarizer 220, compensator 225,variable aperture 230 and first focusing lens 235. First focusing lens235 directs light beam 210 onto film stack 240 on wafer 245 held bywafer stage 250. Light beam 210 reflects from film stack 240 and passesthrough second focusing lens 255, then through interchangeable aperture260, analyzer 265 and into mirror box 270. Beamsplitter 275 in mirrorbox 270 splits beam 210 into two beams, one that is directed totelescope 280 and another directed onto array detector 285.

Not all the components shown need be present in any particularinstrument, and first and second focusing lens 235 and 255 may becompound lens systems. Lens 255 may also be eliminated from somesystems.

The ellipsometer measures two parameters. The first is tan psi, thechange in the amplitude ratio upon reflection. The second is delta, thechange in the phase difference upon reflection of light beam 210. Psiand delta are functions of the optical constants of the surface, thewavelength of the light used, the angle of incidence, the opticalconstants of the ambient medium, and the thickness (t) and opticalconstants (n, k) of film stack 240.

In operation, tan psi and delta are measured at multiple incident anglestheta, and the results compared to a theoretical model of the film orfilm stack being measured.

FIG. 3 shows a configuration of a photoacoustic film thicknessmeasurement system. Photoacoustic system 300 includes an optical/heatsource 302 at the top of the Figure, a laser 342 at the top right, avideo camera 307, a sample stage 306 at the top center, a mirror 360, abeam splitter 304 for directing radiation at the camera, a pump-probebeamsplitter assembly 346, a first acousto-optic modulator 352 adjacentto the beamsplitter assembly, and a second acousto-optic modulator 324at the bottom right. Pump-probe beamsplitter assembly 346 includes awaveplate 348 and a polarized beamsplitter 350. Additionallyphotoacoustic system 300 includes a spatial filter 320 at the lowerleft, a retroreflector 322, a wave plate 316, a polarized beamsplitter314 on the left, and a detector 340 with input 338.

Additionally photoacoustic system 300 includes a linear polarizer 332, afirst lens 308, a second lens 309, a beamsplitter 313, a first positionsensitive detector (PSD) 334, a detector 356, a beamsplitter 354, abeamsplitter 314 and a detector 330. Finally, the photoacoustic system300 includes a detector 310 that is optically connected to beamsplitter312 via sampled probe beam 328. These elements are interrelated as shownin FIG. 2.

Laser 342 is preferably a titanium-sapphire laser operating at 80 MHzand emitting light at a wavelength of 800 nm. The laser can also beconfigured with a frequency doubling crystal to emit light at 400 nm.

In operation, beamsplitter 312 is used to sample the intensity of theincident probe beam in combination with detector 310.

Additionally optical/heat source 302, which functions as a variablehigh-density illuminator, provides illumination for a video camera 307and a sample heat source for temperature-dependent measurements undercomputer control. The video camera 307 provides a displayed image for anoperator or appropriate pattern recognition software, and facilitatesthe setup of the measurement system.

The sample stage 306 is preferably a multiple-degree of freedom stagethat is adjustable in height (z-axis), position (x and y-axes), and tilt(theta), and allows motor controlled positioning of a portion of thesample relative to the pump and probe beams. The z-axis is used totranslate the sample vertically into the focus region of the pump andprobe, the x- and y-axes translate the sample parallel to the focalplane, and the tilt axes adjust the orientation of the stage samplestage 306 to establish a desired angle of incidence for the probe beam.

Beam splitter 304 is a broadband beam splitter that directs video and asmall amount of laser light to the video camera 307. The video camera307 and local processor can be used to automatically position the pumpand probe beams on a measurement site.

The pump-probe beam splitter assembly 346 splits an incident laser beampulse (preferably of picosecond or shorter duration) into pump and probebeams, and includes a rotatable half-waveplate 348 that rotates thepolarization of the incident beam. Waveplate 348 is used in combinationwith polarized beam splitter 350 to effect a continuously variable splitbetween pump and probe power. This split may be controlled by thecomputer by means of a motor to achieve an optimal signal-to-noise ratiofor a particular sample. The appropriate split depends upon factors suchas the reflectivity and roughness of the sample. Adjustment is effectedby having a motorized mount rotate waveplate 348 under computer control.

A first acousto-optic modulator 352 chops the pump beam at a frequencyof about 1 MHz. A second acousto-optic modulator (AOM) 324 chops theprobe beam at a frequency that differs by a small amount from that ofthe pump modulator acousto-optic modulator 352. As will be discussedbelow, the AOMs may be synchronized to a common clock source, and mayfurther be synchronized to the pulse repetition rate (PRR) of the laserthat generates the pump and probe beams.

A spatial filter 320 is used to preserve at its output a substantiallyinvariant probe beam profile, diameter, and propagation direction for aninput probe beam which may vary due to the action of the mechanicaldelay line shown as the retroreflector 322. The spatial filter 320includes a pair of apertures A1 and A2, and a pair of lenses L4 and L5.An alternative embodiment of the spatial filter incorporates an opticalfiber.

Waveplate 316 is a second adjustable half-waveplate that functions, withpolarized beamsplitter 314, in a similar manner to the waveplate 348 andpolarized beamsplitter 350 of the beamsplitter assembly 346. Withwaveplate 316, the intent is to vary the ratio of the part of the probebeam impinging on the sample to that of the portion of the beam used asa reference (input to input 338 of detector 340). Waveplate 316 may bemotor controlled to achieve a ratio of approximately unity. Theelectrical signals produced by the beams are subtracted, leaving onlythe modulated part of the probe to be amplified and processed.

The linear polarizer 332 is employed to block scattered pump lightpolarization, and to pass the probe beam. First and second lenses 308and 309 are pump and probe beam focusing and collimating objectives,respectively. The beamsplitter 313 is used to direct a small part of thepump and probe beams onto a first position sensitive detector 334 thatis used for auto focusing, in conjunction with the processor andmovements of the sample stage 306. The position sensitive detector 334is employed in combination with the processor and thecomputer-controlled sample stage 306 (tilt and z-axis) to automaticallyfocus the pump and probe beams onto the sample to achieve a desiredfocusing condition.

A dual metrology system is described below.

FIG. 4 illustrates a plan view of a dual measurement system formeasuring the thickness of transparent and opaque films by combining aspectroscopic ellipsometer 410 and a photoacoustic thickness system 415.Sharing the laser provides for a substantial reduction in cost.

FIG. 5 shows a plan view of a dual metrology system including aspectroscopic ellipsometer and a photoacoustic film thicknessmeasurement system arranged transversely to one another. Dual metrologysystem 500 comprises ellipsometer incident beam generation assembly 510,ellipsometer measurement assembly 516, photoacoustic incident beamgeneration assembly 520 and photoacoustic measurement assembly 526.

With regard to the ellipsometer, the correspondences between FIG. 5 andFIG. 2 are as follows. Ellipsometer incident beam generation assembly510 corresponds to 205, 215, 220, 225, 230 and 235; ellipsometermeasurement assembly 516 corresponds to 255, 260, 265, 270, 275, 280 and285; ellipsometer light beam 513 corresponds to light beam 210; andwafer 530 corresponds to 240 and 245.

With regard to the photoacoustic system, the correspondences betweenFIG. 5 and FIG. 3 are as follows. Photoacoustic incident beam generationassembly 520 corresponds to all elements in the optical path from samplestage 306 and laser 342 via first lens 308; photoacoustic light beam 523corresponds to excitation beam 362; wafer 530 is not shown in FIG. 3,but is normally placed on sample stage 306; and photoacousticmeasurement system 526 corresponds to the collection of all otherelements and assemblies in FIG. 3.

FIG. 6 shows an alternative embodiment of a dual metrology systemwherein ellipsometer incident beam generation assembly 610 andphotoacoustic incident beam generation assembly 620 are supplied lightfrom the same laser 640. All elements and assemblies in the range of 610to 630 in FIG. 6 are respectively identical to 510 to 530 in FIG. 5.

The dual metrology system 600 in FIG. 6 additionally includes a laser640, switchable mirror (or beam splitter) 644, mirror 648, continuumgenerator 652, and mirror 654. The beam 642 exiting from laser 640becomes either beam 646 or 650, depending on the operation of switchablemirror 644. Beam 646 is directed via mirror 648 to photoacousticincident beam generation assembly 620, and beam 650 is directed viacontinuum generator 652 and mirror 654 to ellipsometer incident beamgeneration assembly 610. Thereby, both ellipsometer incident beamgeneration assembly 610 and photoacoustic incident beam generationassembly 620 are supplied laser light by the same laser light source,i.e., laser 640. The continuum generator 652 in FIG. 6 generates acontinuum from laser 640 so that the beam waist of beam 650 is reduced,thereby reducing the spot size of ellipsometer light beam 613 when itstrikes the measurement spot 632 on wafer 630. In a first embodiment,the continuum is generated by a continuum generator that comprises alens 622 in FIG. 8 that focuses the beam on a crystal 621, e.g.sapphire, that generates a broad frequency spectrum. FIG. 9 shows thespectrum of the beam before and after crystal plate 621, converting anarrowband incident beam peaked at 800 nm to a broadband beam having anintensity distribution substantially uniform from 400 nm through 2microns. In one embodiment, a sapphire plate reduced the beam spot sizeto less than 3 microns is diameter.

The problem addressed by the continuum generator is that ultrafastfemtosecond pulse lasers in the microjoule power range used inmultiple-angle spectroscopic ellipsometers generate a beam having a lowpeak power. Those skilled in the art well appreciate that this low peakpower results in a poor signal to noise ratio. According to one aspectof the present invention, the beam spectrum is broadened, such that thespot size on the target is reduced in diameter and the beam intensity isincreased, without increasing the power of the laser. Advantageously,the decrease in beam spot size permits the measurement of smallergeometries.

Since the continuum generator 652 in the present embodiment makes beam650 a microspot broadband beam, without otherwise affecting operation,continuum generator 652 is optional and could be omitted if theintensity in a conventional beam is acceptable.

In another embodiment of the combined system, continuum generator 652incorporates a short length of commercially available photonic crystaloptical fiber. The purpose of the fiber is to generate a high-brightnessbroadband continuum from laser 640. A lens focuses the laser beam 650onto the input face of the fiber. The continuum output is directed tothe ellipsometer incident beam generation assembly 610 and produces asingle mode output with beam waist defined by the mode field diameter ofthe fiber.

In yet another embodiment of the combined system, continuum generator652 incorporates a short length of commercially available taperedoptical fiber. The fiber is tapered such that the diameter is reduced toa few microns along the central portion of its length. The taperedregion of the fiber will exhibit highly non-linear optical propertiesand will generate a high-brightness broadband continuum from laser 640.A lens focuses the laser beam 650 onto the input face of the fiber. Thecontinuum output is directed to the ellipsometer incident beamgeneration assembly 610 and produces a single mode output with beamwaist defined by the mode diameter of the fiber.

In yet another embodiment of the present invention, ellipsometer 410 ofdual metrology system 400 can also be any other type of transparent filmthickness measurement system, such as a reflectometer.

Method of Operation

The method for measuring the thickness of a film on a wafer using thedual metrology system is described below.

FIG. 7 is a flowchart of a method of measuring the thickness of a filmon a wafer using the dual metrology system. Method 700 includes thefollowing steps:

Step 710: Positioning in Measurement Area a Selected Site on a FilmFormed on a Sample

In this step, controller 420 sends an instruction via communicationlines 430 to robotics & wafer handling system 450 to load a wafer fromcassette station 440 onto measurement stage 460 and to position thewafer such that the site is located in the focal area.

Step 720: Making a Measurement on the Selected Site

In this step, controller 420 sends an instruction via communicationlines 430 to either ellipsometer 410 or photoacoustic system 415,depending on whether the film to be measured on the wafer isrespectively transparent or opaque, to make a measurement at theselected site.

Step 730: Calculating the Film Thickness at the Selected Site Based onthe Measurement

In this step, controller 420 (or either ellipsometer 410 orphotoacoustic system 415) calculates the film thickness at the selectedsite based on the measurement made in step (b). The calculation can beperformed in any unit that is convenient. The ellipsometer orphoto-acoustic system may or may not include a unit with calculationcapability, as the designer prefers.

Step 740: Reporting the Film Thickness Measurement

In this step, either ellipsometer 410 or photoacoustic system 415reports the measurement of the film thickness.

In an alternative method of operation, both ellipsometer 410 andphotoacoustic system 415 are used in steps 720, 730 and 740 to measure,calculate and report the film thickness (refer to FIG. 6). The resultsof both measurements are then used, for example, to calculate thecombined transparent and opaque thicknesses. Further, ellipsometer 410can be used to calculate the transparent film thickness, andphotoacoustic system 415 can be used to calculate the speed of sound inthe film. Further, ellipsometer 410 can be used to calculate the“optical constants” meaning, as used in the field, n (refractive index)and k (absorption coefficient) of an opaque film to improve the modelfor calculating the film thickness using photoacoustic system 415.Further, ellipsometer 410 can be used to calculate the transparent filmthickness, and that result can be used to improve the model forcalculating the density of the film using photoacoustic system 415.

In one embodiment of the invention, both the ellipsometer andphotoacoustic system are used to measure, calculate and report the filmthickness. The two results may be combined to give an improved value forthickness by calculating a weighted average of the results, the weightdepending on the designed thickness of the film.

In another embodiment of the invention, both measurements of theellipsometer and photoacoustic system are used to calculate the combinedtransparent and opaque thicknesses of a material film stack consistingof one or more transparent films lying over one or more opaque films.For example, the photoacoustic system would measure the total thicknessof a film stack. The ellipsometer would measure the thickness of theupper transparent layer(s). Subtraction gives the thickness of theopaque layers.

In another embodiment, the ellipsometer is used to calculate thetransparent film thickness, and the thickness measurement from thephotoacoustic system is used to calculate the speed of sound in thefilm. Measurement of the speed of sound provides information about thematerial hardness under elastic deformation.

In general, a thickness measurement is the product of a formulacontaining several parameters that may be of interest; e.g. hardness,doping level, stoichiometry, etc. Ordinarily, the operator will plug inhandbook values for various parameters. With a system according to theinvention, it is possible to combine two measurements to find aparameter representing the actual film being measured.

In the case of a fabrication process using damascene technology, inwhich a trench is etched in a dielectric, the trench is filled withmetal and the excess metal is removed by chemical-mechanical polishing,there may be excess metal removed from the trenches in some locationsand/or excess dielectric removed in areas between concentrations ofmetal.

The photoacoustic system may be used to measure the metal thickness andthe ellipsometer may be used to measure the thickness of the dielectric(commonly oxide). The thicknesses (or their ratio) may be measuredacross the wafer for quality control.

In another embodiment, the ellipsometer is used to calculate the opticalconstants of an opaque film to improve the model for calculating thefilm thickness using the photoacoustic system. The actual values (n,k)for the particular film being measured are used by the photoacousticsystem, giving a better fit to the data than handbook values that wouldotherwise be used.

In another embodiment, the ellipsometer is used to calculate thetransparent film thickness, and that result is used, as in the previousparagraph, to improve the model for calculating the density of the filmusing the photoacoustic system. The photoacoustic measurement depends onthe sound velocity and density for each film. Since the sound velocityand density may both depend on the same process parameters, the use ofellipsometer results allows a velocity calculation with thephotoacoustic data. This velocity can then be used for the densitycalculation.

It can be appreciated that an aspect of this invention is a method ofmeasuring the thickness of a film on a wafer using a metrology systemincluding both an ellipsometer and a photoacoustic system. The methodcomprises the steps of: (a) positioning in the measurement area aselected site on a film formed on a sample; (b) using one of theellipsometer and photoacoustic system to measure a value of a firstparameter of the film at the selected site; using the other of theellipsometer and photoacoustic system to measure the value of a secondparameter of the film at the selected site; and calculating an outputparameter at the selected site based on said values of said first andsecond parameters, in which the ellipsometer system measures thethickness of a film; the photoacoustic system measures the thickness ofthe film and the measurements are combined as the output parameter in aweighted average incorporating empirical weights.

It can be appreciated that a further aspect of this invention isapparatus for measuring a parameter of a film on a wafer including bothan ellipsometer and a photoacoustic system. The apparatus comprises: (a)positioning means for locating in the measurement area a selected siteon a film formed on a sample; (b) laser means for supplying opticalradiation to both of the ellipsometer and photoacoustic system tomeasure a parameter of the film at the selected site; (c) optical meansin the ellipsometer to measure a value of a first parameter of the filmat the selected site; (d) optical means in the photoacoustic system tomeasure a value of a second parameter of the film at the selected site;and (e) calculation means for calculating an output parameter at theselected site based on said values of said first and second parameters,in which the ellipsometer system includes means for measuring thethickness of a film; the photoacoustic system includes means formeasuring the thickness of said film; and further including means forcombining said measurements as said output parameter in a weightedaverage incorporating empirical weights.

If the spectroscopic ellipsometer employed in the dual metrology systemis a microspot broadband spectroscopic ellipsometer, the noise level ofthe continuum generation could potentially reduce the signal-to-noiseratio, thus affecting measurement accuracy. However, this could beovercome by using a higher power light source, such as by using a poweramplifier in the laser to boost the pulse energy of the light beam.

Another advantage of the invention is that both the ellipsometer andphotoacoustic system may be used to measure, calculate and report thefilm thickness. For example, this technique may be particularly usefulfor the case of thin or semi-transparent films. Such films may berelatively thin, and thus more transparent, at some locations on asample and thicker at other locations. The calculated thickness at anysite may be derived from a weighted combination of thicknessmeasurements from both the ellipsometry and photoacoustic systems.Reported thickness at thinner or more transparent sites may tend to relymore heavily on the ellipsometry result, while thicker sites may relymore heavily on the photoacoustic result.

In the following claims, the term “calculating” means any of using ageneral-purpose computer to operate on measured values generated by theellipsometer and/or the pa, or using an arithmetic controller orcomputer incorporated in one or both of the measurement systems.

Although the invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate thatother embodiments may be constructed within the spirit and scope of thefollowing claims.

1. A method of measuring the thickness of a film on a wafer using ametrology system including both an ellipsometer and a photoacousticsystem comprising: (a) positioning in the measurement area a selectedsite on a film formed on a sample; (b) using one of the ellipsometer andphotoacoustic system to measure a value of a first parameter of the filmat the selected site; (c) using the other of the ellipsometer andphotoacoustic system to measure the value of a second parameter of thefilm at the selected site; and (d) calculating an output parameter otherthan a thickness of the film at the selected site based on said valuesof said first and second parameters.
 2. A method according to claim 1,in which said photoacoustic system further measures a total thickness ofa composite film stack containing at least one transparent layerdisposed above at least one opaque layer; and said ellipsometer measuresthe total thickness of said transparent layer, further comprising alsocalculating a thickness of said at least one opaque layer using themeasured total thicknesses of said composite film stack and saidtransparent layer.
 3. A method of measuring the thickness of a film on awafer using a metrology system including both an ellipsometer and aphotoacoustic system comprising: (a) positioning in the measurement areaa selected site on a film formed on a sample; (b) using one of theellipsometer and photoacoustic system to measure a value of a firstparameter of the film at the selected site; (c) using the other of theellipsometer and photoacoustic system to measure the value of a secondparameter of the film at the selected site: and calculating an outputparameter at the selected site based on said values of said first andsecond parameters, in which said photoacoustic system measures thethickness of a film; and said ellipsometer measures the thickness ofsaid film, whereby at least one optical constant of said film may becalculated as said output parameter.
 4. A method of measuring thethickness of a film on a wafer using a metrology system including bothan ellipsometer and a photoacoustic system comprising: (a) positioningin the measurement area a selected site on a film formed on a sample;(b) using one of the ellipsometer and photoacoustic system to measure avalue of a first parameter of the film at the selected site; (c) usingthe other of the ellipsometer and photoacoustic system to measure thevalue of a second parameter of the film at the selected site; andcalculating an output parameter at the selected site based on saidvalues of said first and second parameters, in which said ellipsometersystem measures the thickness of a film; said photoacoustic systemmeasures the thickness of said film; and said measurements are combinedto calculate the density of said film as said output parameter.
 5. Amethod of measuring the thickness of a film on a wafer using a metrologysystem including both an ellipsometer and a photoacoustic systemcomprising: (a) positioning in the measurement area a selected site on afilm formed on a sample; (b) using one of the ellipsometer andphotoacoustic system to measure a value of a first parameter of the filmat the selected site; (c) using the other of the ellipsometer andphotoacoustic system to measure the value of a second parameter of thefilm at the selected site; and calculating an output parameter at theselected site based on said values of said first and second parameters,in which said ellipsometer system measures the thickness of a film; saidphotoacoustic system measures the thickness of said film; and saidmeasurements are combined as said output parameter in a weighted averageincorporating empirical weights.
 6. A method of measuring the thicknessof a film on a wafer using a metrology system including both anellipsometer and a photoacoustic system comprising: (a) positioning inthe measurement area a selected site on a film formed on a sample; (b)using one of the ellipsometer and photoacoustic system to measure avalue of a first parameter of the film at the selected site; (c) usingthe other of the ellipsometer and photoacoustic system to measure thevalue of a second parameter of the film at the selected site; andcalculating an output parameter at the selected site based on saidvalues of said first and second parameters, in which said ellipsometersystem measures the thickness of a film; said photoacoustic systemmeasures the thickness of said film; and said measurements are combinedto calculate the speed of sound in said film as said output parameter.7. A metrology apparatus for measuring a parameter of a film on a waferincluding both an ellipsometer and a photoacoustic system comprising:(a) positioning means for locating in the measurement area a selectedsite on a film formed on a sample; (b) laser means for supplying opticalradiation to both of the ellipsometer and photoacoustic system tomeasure a parameter of the film at the selected site; (c) optical meansin the ellipsometer to measure a value of a first parameter of the filmat the selected site; (d) optical means in the photoacoustic system tomeasure a value of a second parameter of the film at the selected site;and (e) calculation means for calculating an output parameter other thana thickness of the film at the selected site based on said values ofsaid first and second parameters.
 8. An apparatus according to claim 7,in which said photoacoustic system includes means for measuring a totalthickness of a composite film stack containing at least one transparentlayer disposed above at least one opaque layer; and said ellipsometerincludes means for measuring the total thickness of said transparentlayer, further comprising also calculating a thickness of said at leastone opaque layer using the measured total thicknesses of said compositefilm stack and said transparent layer.
 9. An apparatus according toclaim 7, in which said photoacoustic system includes means for measuringthe thickness of a damascene metal; said ellipsometer includes means formeasuring the thickness of a dielectric film separating sections of saiddamascene metal; and said output parameter further comprises athickness-related parameter that represents the relationship betweensaid thicknesses.
 10. An apparatus according to claim 7, in which saidellipsometer includes means for converting an input narrow spectrum beamto a broadband beam that is focused on said selected site.
 11. Anapparatus according to claim 10, in which said means for convertingincludes a lens that focuses said beam on a crystal.
 12. An apparatusaccording to claim 10, in which said means for converting includes alens that focuses said beam into a photonic crystal optical fiber. 13.An apparatus according to claim 10, in which said means for convertingincludes a lens that focuses said beam into a tapered single modeoptical fiber.
 14. An apparatus for measuring a parameter of a film on awafer including both an ellipsometer and a photoacoustic systemcomprising: (a) positioning means for locating in the measurement area aselected site on a film formed on a sample; (b) laser means forsupplying optical radiation to both of the ellipsometer andphotoacoustic system to measure a parameter of the film at the selectedsite; (c) optical means in the ellipsometer to measure a value of afirst parameter of the film at the selected site; (d) optical means inthe photoacoustic system to measure a value of a second parameter of thefilm at the selected site; and (e) calculation means for calculating anoutput parameter at the selected site based on said values of said firstand second parameters, in which said photoacoustic system includes meansfor measuring the thickness of a film; and said ellipsometer includesmeans for measuring the thickness of said film, whereby at least oneoptical constant of said film may be calculated as said outputparameter.
 15. An apparatus for measuring a parameter of a film on awafer including both an ellipsometer and a photoacoustic systemcomprising: (a) positioning means for locating in the measurement area aselected site on a film formed on a sample; (b) laser means forsupplying optical radiation to both of the ellipsometer andphotoacoustic system to measure a parameter of the film at the selectedsite; (c) optical means in the ellipsometer to measure a value of afirst parameter of the film at the selected site; (d) optical means inthe photoacoustic system to measure a value of a second parameter of thefilm at the selected site; and (e) calculation means for calculating anoutput parameter at the selected site based on said values of said firstand second parameters, in which said ellipsometer system includes meansfor measuring the thickness of a film; said photoacoustic systemincludes means for measuring the thickness of said film; and furtherincluding means for combining said measurements to calculate the densityof said film as said output parameter.
 16. An apparatus for measuring aparameter of a film on a wafer including both an ellipsometer and aphotoacoustic system comprising: (a) positioning means for locating inthe measurement area a selected site on a film formed on a sample; (b)laser means for supplying optical radiation to both of the ellipsometerand photoacoustic system to measure a parameter of the film at theselected site; (c) optical means in the ellipsometer to measure a valueof a first parameter of the film at the selected site; (d) optical meansin the photoacoustic system to measure a value of a second parameter ofthe film at the selected site; and (e) calculation means for calculatingan output parameter at the selected site based on said values of saidfirst and second parameters, in which said ellipsometer system includesmeans for measuring the thickness of a film; said photoacoustic systemincludes means for measuring the thickness of said film; and furtherincluding means for combining said measurements as said output parameterin a weighted average incorporating empirical weights.
 17. An apparatusfor measuring a parameter of a film on a wafer including both anellipsometer and a photoacoustic system comprising: (a) positioningmeans for locating in the measurement area a selected site on a filmformed on a sample; (b) laser means for supplying optical radiation toboth of the ellipsometer and photoacoustic system to measure a parameterof the film at the selected site; (c) optical means in the ellipsometerto measure a value of a first parameter of the film at the selectedsite; (d) optical means in the photoacoustic system to measure a valueof a second parameter of the film at the selected site; and (e)calculation means for calculating an output parameter at the selectedsite based on said values of said first and second parameters, in whichsaid ellipsometer system includes means for measuring the thickness of afilm; said photoacoustic system includes means for measuring thethickness of said film; and further including means for combining saidmeasurements to calculate the speed of sound in said film as said outputparameter.
 18. A metrology apparatus for measuring a parameter of atleast one film on a wafer including both an ellipsometric system and aphotoacoustic system comprising: at least one laser for supplyingoptical radiation to at least one of said ellipsometric system and tosaid photoacoustic system for use in measurement of a first parameterand a second parameter, respectively, of the at least one film at aselected site; and a processor having inputs coupled to an output ofsaid ellipsometric system and to an output of said photoacoustic system,and responsive to measurement values of first and second parametersoutput therefrom, to calculate an output parameter, other than athickness of said at least one film, at the selected site based on acombination of said measurement values of said first parameter andsecond parameter.
 19. A metrology apparatus according to claim 18, inwhich said ellipsometric system comprises an optical converter toconvert an input narrow spectrum laser-originated beam to a broadbandbeam that is directed to said selected site.
 20. An apparatus accordingto claim 19, where said converter comprises a lens that focuses saidbeam on a crystal.
 21. An apparatus according to claim 19, where saidconverter comprises a lens that focuses said beam into an end of anoptical fiber.
 22. An apparatus according to claim 19, where saidconverter comprises a lens that focuses said beam into an end of atapered optical fiber.